I think the only problem is that if two Doctors turn up at a fixed point then surely they can't do anything to change the situation around them?
I'm all in favour of multi-Doctor stories as a bit of a celebration, but they do tend to lack something in the narrative sense. I guess it's the difficulty of having multiple lead characters, all with essentially the same personality characteristics. The one they briefly mounted in 1992 sounds like a good idea (benevolent monster changes history to keep the Fourth Doctor alive when he should have regenerated, the knock on effect adjusts the paths of all future Doctors) except that it reduced all but Tom Baker to cameo appearances.